1. Field of the Invention
This invention relates to the servo controller operating at least the voice coil of a voice coil actuator in a hard disk drive.
2. Background Information
Modern hard disk drives include a servo controller driving at least a voice coil actuator to position a read-write head near a track on a rotating disk surface. The read-write head communicates with the servo controller, providing feedback, which is used in controlling the read-write head's positioning near the track. The read-write head is embedded in a slider, which floats, on a thin air bearing, a very short distance above the rotating disk surface.
A voice coil actuator typically includes a voice coil, which swings at least one actuator arm in response to the servo controller. Each actuator arm includes at least one head gimbal assembly typically containing a read-write head embedded in a slider. The head gimbal assembly couples through a load beam to the actuator arm in the voice coil actuator. The read-write heads mount on head gimbal assemblies, which float on the thin air bearing off the hard disk drive surface when in operation. The air bearing is formed by the rotating disk surface and the slider attached to the head gimbal assembly.
A hard disk drive may have one or more disks. Each of the disks may have up to two disk surfaces in use. Each disk surface in use has an associated slider, with the necessary actuator arm. One actuator arm typically supports one or two sliders. Hard disk drives typically have only one voice coil actuator.
Often there is one slider for a given hard disk drive surface. There are usually multiple heads in a single hard disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet which interacts with a time varying electromagnetic field induced by a voice coil. This provides a lever action via an actuator axis. The lever action acts to move the actuator arm(s), positioning the head gimbal assembly(ies) over specific tracks with speed and accuracy. Actuators often include the voice coil, the actuator axis, the actuator arms and the head gimbal assemblies. An actuator may have as few as one actuator arm. A single actuator arm may connect with two head gimbal assemblies, each with at least one head slider.
Typically, a slider is rigidly attached to a head suspension to make a head gimbal assembly. The attachment of the slider is through a flexure, which provides an electrical interconnection between the read-write head in the slider and the disk controller.
The hard disk drive controller controls the voice coil to position the read-write head over a track above a rotating disk surface. In a typical hard disk drive control system, the state -space controller/observer (or estimator) design is frequently used. This approach has advantages, such as effective filtering of position and velocity, use of an estimation error to handle servo defects, etc. A state space controller is generally regarded as a control mechanism exerting a control, based upon feedback and an internal state. The feedback often includes a Position Error Signal (PES).
These state-space or discrete-time state estimators are typically implemented in one of two forms. One form is the so-called prediction estimator that estimates the state variable based on the plant output and control of the previous sampling period. The other form is called the current estimator, which comes from the use of the current time measurement to estimate the state variables. The plant as used herein refers to the voice coil actuator interacting with the rotating disk surface(s) to create the measured current read-write head position Ycur.
Prediction estimator equations have a more direct relationship with the internal dynamics of the hard disk drive. The prediction estimator equations are a discrete time step form of a continuous-time state observer model. However, the current estimator equations can be derived by means of a discrete Kalman filter formalism with a predetermined knowledge of the current measurements. Today, the current estimator is the predominant hard disk drive servo control model. This is due to the belief that its estimates are more reliable for small computational delays.
As the track density (measured tracks-per-inch, or TPI) of hard disk drives increases, the requirement for servo positioning accuracy increases. This requirement forces the sampling rate to also be increased. At the same time, more sophisticated control algorithms need to be used to minimize the Track Mis-Registration (TMR) as the track density, TPI, increases.
The inventors have discovered that a number of previously held and unquestioned assumptions regarding at least computational delays are no longer true. The computation time delay is no longer negligibly small. It needs to be accounted for in the design of the servo controller.
A portion of each hard disk drive's accessible data is reserved to identify the read-write head location. Increasing the sampling rate above certain thresholds requires reducing the data available for use by a computer accessing the hard disk drive, which reduces its market value. This market constraint significantly limits the sampling frequency, while at the same time, the TPI continues to grow.
Control systems need to account for these new and previously insignificant realities in hard disk drives, without unnecessarily increasing the data overhead to support increased sampling rates.